Polyethylenes are categorized according to their densities, which are often used as a guide to end-use applications. For example, high density polyethylene (HDPE) has a low degree of branching, which results in a compact structure having high tensile strength. HDPE is used in products such as pipes and drums. Medium density polyethylene (MDPE) has a high degree of chemical resistance as well as shock and drop resistance, and is used in products such as shrink film. Low density polyethylene (LDPE) possesses random long chain branching, with “branches on branches”. LDPE can provide good resistance to high temperatures and impact, and has been used in applications such as cling film and squeezable bottles. Linear low density polyethylene (LLDPE) has an essentially linear structure but also has low density because of its short chain branching, and is used in applications such as stretch film and coatings for cables.
Various processes can be used to produce polyethylene, including gas phase processes, solution processes, and slurry processes. In ethylene slurry polymerization processes, diluents such as hexane or isobutane may be used to dissolve the ethylene monomer, comonomers and hydrogen, and the monomer(s) are polymerized with a catalyst. Following polymerization, the polymer product formed is present as a slurry suspended in the liquid medium.
In typical multi-reactor cascade processes, such as those disclosed, e.g., in WO 2012/028591 A1, U.S. Pat. No. 6,204,345 B1, and WO 2005/077992 A1, monomer(s), hydrogen, catalyst and diluent are fed into the first of three reactors where a slurry forms from the polymer particles contained within the diluent and unreacted monomer. The reactors can be operated in parallel or in series, and the types/amounts of monomer and conditions can be varied in each reactor to produce a variety of polyethylene materials, including unimodal (molecular weight distribution) or multimodal polyethylene material. Such multimodal compositions are used in a variety of applications; e.g., WO 2012/069400 A1 discloses trimodal polyethylene compositions for blow moldings.
A difficulty occasionally encountered by continuous stirred tank reactors in slurry polymerization systems is blockage of the inlets to the slurry pumps. This can occur if polymeric material that has accumulated on the reactor baffles, mechanical supports for the baffles, or the inner walls of the reactor suddenly become dislodged and flow with the reactor slurry to the slurry pump inlet. The dislodged material are lumps of polymer that quickly block the pump suction when the polymer lumps arrive at the pump inlet. The polymer lumps can build up over time from fouling that occurs when the slurry returned from an external slurry cooler is discharged into the vapor space above the liquid level in the reactor. Upon discharge, liquid in the slurry separates from the solids, i.e. flashes, leaving behind the sticky lumps of polymer that can adhere to the baffles, baffle support cross pieces and the reactor inner wall surfaces. In addition, reactor splashing may occur when the returned slurry drops onto the liquid surface of the reactor contents, depositing solids onto the exposed metal surfaces. Repeated deposition can cause the polymeric material to gradually grow into lumps of polymer adhered to the metal surfaces. These large polymer lumps eventually may fall off due to cycling of the reactor liquid level or drying of the polymer on the metal surfaces inside the reactor, causing a loss of adhesion. However, in either case, the polymer lumps eventually flow to the slurry pump, block the pump impeller and sharply reduce the flow of slurry through the slurry cooler. Blockage of the slurry pumps in turn can cause operational interruptions since the pumps must be isolated and cleaned.
Baffled polymerization reactors have been used in olefin polymerization reaction systems. For example, WO 2009/142730 A1 and U.S. Pat. No. 8,410,230 B2 relate to an olefin polymerization process for producing polymers having broadened molecular weight using internal baffles that separate the reactor zones. U.S. Pat. No. 7,214,750 B2 discloses a process for producing polymers in a reactor using tubular heat-exchanger bundles as baffles in the periphery of the vessel, and U.S. Pat. No. 7,993,593 B2 discloses an olefin polymerization reactor having a tubular baffle containing a gas inlet orifice. Nonetheless, a continuing need exists for an ethylene slurry polymerization processes having improved slurry pump performance through reduced pump blockage.